It was the aim of our investigation to evaluate the treatment efficacy of three predefined tooth movements (translation, rotation and incisor torque) with aligners using the Invisalign® system, with respect to the influence of attachments/Power Ridges, the staging and the patients’ compliance.
In our study, the overall efficacy amounted to 59.3%. It should be noted, however, that the total efficacy in our study was composed of the efficacy of the three investigated movements: premolar derotation, molar distalization and incisor torque. Thus, it does not reflect the efficacy of complete orthodontic treatment.
Some authors doubt that bodily movements (especially incisor torque) can be accomplished using removable plastic appliances . To generate the needed force systems, Invisalign® provides the use of an attachment or Power Ridge. As the results of our study indicate, both are practicable; nevertheless, a loss of torque up to 50% must be considered. However, it must be noted that the efficacy of fixed orthodontic appliances does not reach 100% either: Conventional orthodontic brackets and wires do not completely fill the bracket slots, so that the wire is able to twist, leading to a loss of moment, known as the so-called “torque play”. Moreover, the size and quality of the wire, the wire edge bevelling, the bracket material (polycarbonate brackets vs. metal and ceramic brackets) and bracket design, the interbracket distance, the vertical positioning of the bracket as well as the mode of ligation all influence the torque movement of conventional fixed appliances . Due to this large amount of variation, it is almost impossible to calculate how much loss of torque expression exists with the use of a fixed appliance.
As described in the literature, one of the most difficult movements to perform with an aligner is the derotation of a cylindric tooth, as thermoplastic appliances tend to lose anchorage and slip off due to the presence of few undercuts and a round tooth shape [12, 13]. This is reflected by the published results for premolar and canine derotation, which range between 29.1% to 49.7% [6, 14–16]. In our study, the mean accuracy for premolar derotation (group 2) was 42.4% without and 37.5% with the support of an attachment. The lower efficacy in the group supported with an attachment was mainly due to poor patient compliance, which significantly reduced the treatment efficacy. It seems that if the aligner fitting is reduced but there is no attachment on the tooth’s surface, the rotational force transfer just decreases, whereas with an attachment, counter-moments can occur, leading to tooth movement in the opposite direction. If one was to exclude the patient’s poor compliance, a mean accuracy of 47.3% would be achieved. Overall, the amount of derotation influenced the accuracy significantly: If rotations greater than 15° were attempted, the mean accuracy of premolar derotation decreased by 46%, from 43.3% to 23.6%. These results were in accordance with those of Kravitz et al., who reported a significant reduction of up to 52.5% in the accuracy of canine derotation for rotations greater than 15° . In addition to the amount of derotation, the staging (amount of derotation/aligner) also has a considerable impact on the treatment efficacy: for premolar derotations with a staging <1.5°/aligner, the total efficacy was 41.8% (SD = 0.3), whereas with a staging >1.5°/aligner, the accuracy decreased to 23.2% (SD = 0.2).
Among clinicians, one very important aspect is if and to what extent anterior-posterior movements can be performed using RTAs because this significantly increases the indications and allows for usage in even more complex malocclusions. Some authors reported a low accuracy of Invisalign® in correcting large anterior-posterior discrepancies . To date, no scientific study has evaluated the exact efficacy of molar distalization using RTA. In our study, the molar distalization revealed the highest accuracy, approximately 87%. None of the patients used class II elastics during treatment. However, it should be noted that we measured the accuracy of distalization using a maximal amount of desmodontal anchorage: no anterior teeth were moved during the distalization of single molars. Furthermore, the anchorage lost in the posterior region during the retrusion of anterior teeth was not considered because the impressions were taken directly after the distalization of the second/first molar (T2). It remains to be investigated what impact simultaneous distalization of anterior teeth has on the overall efficacy of molar distalization, if the use of interarch elastic enhances distalization, and what amount of anchorage lost in the posterior region occurs during the retrusion of anterior teeth.
This study exhibited some limitations:
Because the data from the final tooth position in the ClinCheck® did not show the palatal surface, we used the untreated teeth as reference points for superimposition. Although only one tooth per hemiarch was moved, leaving enough teeth as a reference structure, relative movements of the reference teeth could not be excluded due to periodontal anchorage.
Furthermore, the aligner material we used in our study was the so-called the Exceed30 (EX30), the original aligner material from Align Technology. From the first quarter of 2013, a new aligner material called SmartTrack™ (LD30) was introduced to the orthodontic market by Align Technology. To what extent the new aligner material influences the treatment efficacy needs to be investigated.
Our evaluation focuses on the treatment efficacy of the three tooth movements during a certain set of aligners (on average 18) because during regular orthodontic treatment, the amount of aligners used to treat patients’ malocclusion is greater. In turn, the overall efficacy may be greater as the tooth movements are performed more slowly throughout the entire treatment time.
Another methodological deficit of this study was the low number of study participants, recruited from one single orthodontic practice. The treatment outcome using the Invisalign® appliance is strongly influenced by the experiences of the clinicians, so that the study results are not generally valid. To provide more accurate results on the treatment efficacy, a follow-up study with a larger sample size from several orthodontists would be useful.
Finally it must be said that we only investigated the efficacy of orthodontic treatment using the Invisalign® system with regards to the influence of auxiliaries (Attachment/Power Ridge), the staging (movement per aligner), as well as patient’s compliance. No comparison was made between the Invisalign® system and other orthodontic systems such as conventional fixed appliances, lingual appliances or other removable thermoplastic appliance systems. Further studies should compare treatment efficacy between different orthodontic treatment systems to find out which system is most appropriate for different dental malocclusion.